A variable transmission is mainly used for self-propelled machinery, such as a propelled lawn mower. The traditional variable transmission operates using a friction ring or a friction plate. For example, US Patent Publication No. US2003/0006074 A1 discloses a variable transmission which comprises a driving member on which a driving friction ring is installed, and a driven member on which a driven friction ring is arranged. The power of the driving member comes from a belt pulley connected with a transmission belt; the driven friction ring axially moves to a position engaging with the driving friction ring during engaging and disengaging, the driving friction ring transfers the power to the driven friction ring, and the driven friction ring drives an output shaft to rotate. Such a transmission is simple in structure and low in cost, but lacks reliability, and the transmission ratio cannot be ensured because a driving mechanism and a driven mechanism are likely to slide relatively. When the self-propelled machinery meets a relatively great resistance, the driving mechanism and the driven mechanism are likely to slide relatively, so that the reduction of the strength and the resistance of parts, even the deformation of the parts, will be caused due to a large amount of heat energy generated by friction. However, if a gear transmission device is directly used as a clutch, when the clutch is engaged with a driving device rotating at a high speed, a gear of the clutch is likely to wear due to impact. Such phenomenon is referred to as “gear interfering”, and the more the “gear interfering” phenomena that exists during engaging and disengaging of the gear, the more serious the wear is, thereby resulting in poor or failure transmission operation.
In order to solve the above problems of a friction clutch and a gear clutch, disclosed in US Patent Publication No. US2012/0145501 A1 is a variable speed transmission which is provided with a speed reduction mechanism, wherein the speed reduction mechanism comprises a worm 5 and a worm gear 6 matched with the worm 5 (refer to FIG. 1 in the patent). The worm gear is provided with driving teeth and can be installed on an output shaft 2 in a freely rotating manner. The worm gear is provided with a friction clutch mechanism 7 and a gear engaging and disengaging mechanism 8. The friction clutch mechanism comprises a conical friction ring 7A fixedly connected with the output shaft 2 and a conical friction ring 7B which rotates in synchronization with the worm gear. The gear clutch mechanism 8 can be shifted to an engaging position axially from a disengaging position by a clutch shift. The gear clutch mechanism 8 is internally provided with a grooved structure for accommodating U-shaped pins. The output shaft 2 is provided with U-shaped pin inserted holes. When the gear clutch mechanism 8 and the worm gear 6 are meshed, the U-shaped pins transfer the power to the output shaft 2. A spring 10 is provided on the output shaft, and axially moves along with the gear clutch mechanism 8. The end part of the spring props against the middle of the worm gear. The worm gear 6 axially shifts, such that the conical friction rings 7A and 7B are engaged (refer to FIG. 4 and FIG. 5), and at this moment, the output shaft 2 realizes pre-transmission under the action of the friction rings. The gear clutch mechanism 8 continuously moves to a position meshing with the worm gear 6, and the spring 10 enters into a cavity of the gear clutch mechanism 8. At this moment, reliable gear transmission between the worm 5 and the output shaft 2 is realized. Relative to a speed reduction mechanism which only depends on friction transmission, this solution not only ensures the transmission ratio of the variable transmission, but also reduces the impact force generated by the variable transmission during engaging and disengaging. This technical solution has the following defects: the friction clutch mechanism 7 needs to realize engaging and disengaging by via the axial movement of the spring 10, so that the structure is complicated; and the friction clutch mechanism 7 is arranged outside the worm gear, and when the friction engaging and disengaging mechanism 7 is engaged and disengaged, the worm gear also needs to axially move.
Therefore, the present disclosure aims to provide a novel speed reduction mechanism which can reduce the impact generated by clutches during engaging and disengaging, and also can ensure a transmission ratio, especially can simplify the structure of a variable transmission.